Effect of Coordination and Symmetry on the IR spectra of Donor Molecules

Cyano Complexes

CN- is isoelectronic with CO and NO+. Both CO and CN- co-ordinate to the metal atom through carbon. The free CN- ion has a stretching frequency of 2080 cm-1, in complexes it generally has a higher value and a sharp absorption in the range 2000-2200 cm-1 is observed.

Cyano complex

In the free state the contribution of II a is greater than I a towards the resonance hybrid.
In complexes the contribution of I b or II b depends on the metal atom to which it is coordinated. It has been found that CN stretching frequency in its complexes depends on electro negativity, oxidation state and co-ordination number of the metal atom.

i) Electro negativity of the metal The stretching frequency of the ligand CN increases with increase in electro negativity of the metal ion.
[ Ni (CN)4]2-< [ Pd (CN)4 ]2-< [ Pt (CN)4 ]2-
2135 cm-1, 2143 cm-1 2150 cm-1

ii) Oxidation state The CN stretching frequency increases with increase in the oxidation state of the metal atom. Higher oxidation state of the metal would decrease electron density at the metal center for back bonding with the ligand.
[ Ni (CN)4]4-< [ Ni (CN)4]2-
1985 cm-1, 2135 cm-1.

ii) Coordination number The stretching frequency for CN decreases with increase in coordination number of the metal atom.
Increase in Coordination number results in a decrease in positive charge on the metal, which in turn weakens the M-CN sigma bond.

[ Ag (CN)4 ]3-
[ Ag (CN)3 ]2-
[ Ag (CN)2 ]-
2092 cm-1
2105 cm-1
2135 cm-1
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Compariskon of Raman and IR
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